The present invention relates to a conductive adhesive used for solder-free mounting of electronic components and a structure connected by using the same.
In recent years, due to the increased environmental consciousness, the electrical industry now faces the movement to abolish totally lead-containing solder used for mounting of electronic components, and this movement is becoming significant.
As for lead-free mounting techniques, mounting techniques using lead-free solder have been developed keenly and a part of the development has come into practical use. However, still a number of problems remains to be solved, such as influence of a high mounting temperature on low heat-resistant components or lead-free electrodes.
On the other hand, only a few examples of lead-free mounting with the use of a conductive adhesive has been reported so far, which has the following advantages besides the aspect of lead-free mounting.
First, the processing temperature of around 150xc2x0 C. is lower than the temperature for soldering, and electronic components with higher performance can be realized with low cost. Secondly, the specific gravity of a conductive adhesive is about half that of solder, so that electronic equipment can be lightened more easily. Thirdly, since the connection is not achieved by means of metal as in soldering, metal fatigue does not occur, and the reliability of mounting is excellent.
Therefore, it is expected that a revolutionary mounting process that fulfills the needs of environment, low cost, and high reliability can be realized by completing the mounting technique using a conductive adhesive.
The problem with the use of a conductive adhesive for mounting is that the adhesive strength is lower than that of solder. In particular, the strength against bending stress is about {fraction (1/10)} of that of solder, so that an electronic component with a large area to which bending stress easily is applied sometimes suffers from the separation of the electrode with the conductive adhesive at the interface, thereby causing connection failures.
Numerous attempts to improve the adhesive strength have been reported, but not even one technique is capable of achieving the same strength as that of solder. One representative example will be shown below.
As described in the publication supervised by Hiroo Miyairi, xe2x80x9cDevelopment of Functional Adhesives and the New Technologyxe2x80x9d (edited by CMC, Jun. 30, 1997, 194 pages), for example, a number of techniques to improve the adhesive strength by adding an organic metal called a silane coupling agent into the adhesive material, which can form a chemical bond with both resin and metal, has been reported.
However, the aforementioned techniques utilize either a dehydration reaction or a substitution reaction, so that the reactivity of the coupling agent with resin or with metal was poor, and the conditions (temperature, concentration of hydrogen ion, etc.) for optimizing the reaction could not be determined clearly, and so forth. Therefore, a considerable improvement of the adhesive strength was difficult to be achieved.
Furthermore, JP9(1997)-176285A proposes the use of a resin with a phosphoric ester group introduced into the skeleton as a binder resin. According to this method, the functional group in the binder resin is adsorbed to the metal, so that some improvement of the adhesive strength can be achieved. However, the adsorptive power is poor in comparison with a covalent bond or a coordinate bond, so that considerably improved effects could not be obtained.
As described above, the improvement of the adhesive strength has been the key factor to make the practical use of the mounting technique using a conductive adhesive.
It is an object of the present invention to solve the conventional problems described above by providing a conductive adhesive having considerably improved adhesive strength and higher reliability against bending stress. Another object of the present invention is to provide a structure connected by using this conductive adhesive.
To achieve the above object, a conductive adhesive of the present invention includes a binder resin and a metal filler as main components, wherein the binder resin contains a functional group in its molecular chain that forms a multidentate bonding with an electrode metal after the binder resin is adhered.
A connection structure of the present invention is formed by using a conductive adhesive to connect the adhesive with an electrode electrically, wherein the conductive adhesive includes a binder resin and a metal filler as main components, and the binder resin contains a functional group in its molecular chain that forms a multidentate bonding with an electrode metal after the binder resin is adhered.
In the present invention, the multidentate bonding refers to a state in which multidentate ligands (a plurality of chelating ligands) introduced into the binder resin form coordinate bonds with the electrode metal. In other words, the adhesion is not achieved only by using the ordinary weak van der Waal""s power by hydrogen bonding, but instead, a chemical bond (coordinate bond) is formed between the binder resin and the electrode.
The method of introducing a multidentate ligand into a binder resin will be explained by way of the following embodiments.
Embodiment 1
In the first method, a resin into which a desired multidentate ligand was introduced was used as an additive component in the binder resin (a reactive thinner, a hardener, or the like).
For example, a linear epoxy resin with a molecular chain into which a dicarbonyl group expressed by the chemical formula 1 below was introduced is mixed with a ring-opening catalyst, which then is applied to the surface of an electrode (Cu foil). When the resin is heated and hardened, the dicarbonyl group in the central part of the molecule forms a coordinate bond with the electrode (Cu foil) expressed by the chemical formula 2 below. Naturally, the epoxy rings at the both ends of the molecule open and form bridge bonds. 
The binder resin used here includes as the main component an ordinary epoxy resin without any ligand (bisphenol A, bisphenol F, a novolak epoxy resin). The above resin into which the multidentate ligand was introduced is used by mixing and kneading so as to be contained in the binder resin in an amount between 10 and 50 wt %.
Furthermore, a ligand can be introduced into a resin to be used as a hardener in the binder resin, not only for the reactive thinner.
Embodiment 2
In the second method, a resin into which a desired multidentate ligand was introduced was used as the main component (the component contained in the largest amount) in the binder resin.
For example, a bisphenol F-type epoxy resin with a molecular chain into which a dicarbonyl group expressed by the chemical formula 3 below was introduced is mixed with a ring-opening catalyst, which then is applied to the surface of an electrode (Cu foil). When the resin is heated and hardened, the dicarbonyl group in the central part of the molecule forms a coordinate bond with the electrode (Cu foil) as in Embodiment 1. 
The binder resin used here includes as the accessory component an ordinary epoxy resin without any ligand (a reactive thinner, a hardener, or the like). The above resin into which the multidentate ligand was introduced is used by mixing and kneading so as to be contained in the binder resin in an amount between 30 and 100 wt %.
In the adhesive and the connection structure of the present invention described above, it is preferable that the multidentate bonding is formed in a number between 2 and 4. Naturally, the number of the multidentate bonding may be larger.
Furthermore, in the adhesive and the connection structure described above, it is preferable that the resin containing a functional group in its molecular chain that forms a multidentate bonding is present in an amount between 10 and 100 wt % of the total resin.
Furthermore, in the adhesive and the connection structure described above, it is preferable that, taking the conductive adhesive as 100 wt %, the binder resin is contained in an amount between 6 and 25 wt %, and the metal filler is contained in an amount between 75 and 95 wt %. Besides, if necessary, a hardener, a hardening catalyst, a crosslinking agent, and a ring-opening catalyst for the binder resin, a dispersing agent for the metal filler, a viscosity modifier, a pH modifier, or the like may be added optionally. Therefore, in the present invention, xe2x80x9cmain componentsxe2x80x9d in the xe2x80x9cincluding a binder resin and a metal filler as main componentsxe2x80x9d refers to the constitution in which the binder resin and the metal filler together comprise at least 90 wt % of the conductive adhesive.
Furthermore, in the adhesive and the connection structure described above, it is preferable that at least two functional groups are present, which may be the same or different, selected from the group consisting of a carbonyl group, a carboxyl group, an amino group, an imino group, an iminoacetic acid group, an iminopropionic acid group, a hydroxyl group, a thiol group, a pyridinium group, an imido group, an azo group, a nitrilo group, an ammonium group and an imidazole group.
Furthermore, in the adhesive and the connection structure described above, it is preferable that the metal filler is at least one particle selected from the group consisting of silver, silver-plated nickel and silver-plated copper. When the surface of the metal filler is silver, it does not react with the ligand of the binder resin but the ligand reacts selectively with the electrode metal. However, when the ligand of the binder resin is contained in a large amount, even if copper is used as the metal filler, for example, since not all the ligands react with the metal filler, copper also can be used as the metal filler.
Furthermore, in the adhesive and the connection structure described above, it is preferable that the binder resin is at least one resin selected from a thermoplastic resin and a thermosetting resin.
Furthermore, in the adhesive and the connection structure described above, it is preferable that the thermoplastic resin is at least one resin selected from the group consisting of a polyester resin, a silicone resin, a vinyl resin, a vinyl chloride resin, an acrylic resin, a polystyrene resin, an ionomer resin, a polymethylpentene resin, a polyimide resin, a polycarbonate resin, a fluororesin and a thermoplastic epoxy resin.
According to the invention described above, the mounting technique by the conductive adhesive with considerably improved adhesive strength can be realized.
When the conductive adhesive of the present invention and the electrode metal contact each other, the ligand of the binder resin very easily reacts with the metal, so that the ligand is coordinated quickly with the electrode metal to form a chelating ligand, i.e. a strong chemical bonding. Since the ligand is bonded to the molecular chain of the resin, the bonding also is strengthened between the binder resin and the electrode metal as well as between the conductive adhesive and the electrode metal.
The connection structure of the present invention is formed by electrically connecting the conductive adhesive of the present invention with the electrode, so that this connection structure has improved adhesive strength than conventional connection structures. Moreover, since the thermoplastic resin has excellent flexibility in comparison with a thermosetting resin, this conductive adhesive can achieve a bonding with excellent stress relaxation capability against bending stress.
The connection structure of the present invention preferably is formed by mounting a component and a substrate by using the conductive adhesive described above, which can improve the adhesive strength against bending stress even more.
The present invention can be used in place of the conventional solder, for example, as a conductive adhesive to bond a semiconductor substrate with an electronic component chip. Furthermore, the conductive adhesive also can be applied to a conductive paste by filling the conductive adhesive into through holes made in an electrical insulating base material so as to achieve electrical continuity in the thickness direction of the electrical insulating base material.